Imaging spectrometers are adapted to form pictures of a scene in different wavelength bands. For example a visual scene image may be formed at a slit which passes a segment of the image scene for dispersal into its various electromagnetic radiation component wavelengths, i.e. spectral bands. The energy of each of the dispersed spectral bands is typically detected by a two-dimensional detector array. One dimension of the array determines spatial information whereas the orthogonal dimension detects spectral energy information. As the spectrometer is scanned in a direction normal to the slit the entire image scene is serially scanned and the energies of the spectral bands as well as spacial details are detected by the detector. At the detector array the received energy of the bands is dispersed over the array which converts intensity into digitized signals which are, in turn, processed by the overall spectrometer imaging system.
Grating type dispersers as well as prisms have been used to disperse the image segment into component wavelengths for detection. Particularly where gratings are used, a significant degree of polarization sensitivity is imparted into the dispersed, spectral bands. This imparted polarization sensitivity varies with wavelength and is typically greatest at shorter wavelengths. For visible and near visible light, the degree of polarization sensitivity imparted by a grating at wavelengths of approximately 400 nm is on the order of between 2-2.5% for a grating frequency of about 40 grooves per millimeter. The polarization sensitivity is affected by the coating material ruled onto the grating plus any other protective coatings, angle of incidence and grating frequency (number of lines/mm).
Where the energy of the received image is to be measured and compared in the frequencies of the bands for purposes of quantitatively assessing the scene, the grating imparted polarization affects the accuracy of the spectrometer for polarized scene, particularly at the shorter wavelengths. For example, for satellite based oceanographic study, electromagnetic radiation as visible and near visible light reflected from the oceans is polarized to a degree. Measurement of reflected scene energy would therefore be affected by the disperser (e.g., grating) induced polarization, depending upon the orientation of the spectrometer and its dispenser relative to the scene polarization. If the disperser, for example, is disposed such that its imparted polarization is parallel to the scene polarization, band and resultant signals intensities at the array would be greater than if the disperser was positioned such that its imported polarization was in a plane perpendicular to the scene polarization, particularly at shorter wavelengths. Accordingly, the ability to accurately measure scene energies including any scene imparted polarizations is affected by the disperser imparted polarization.
There is, therefore, a need for a device and method to compensate and substantially correct for polarization which is imparted by the spectrometer disperser so that the spectrometer band signal intensities are not as sensitive to the relative orientation of the disperser to any scene polarization. More particularly there is a need for a simple device and method for substantially compensating for diffraction grating imparted polarization for an imaging spectrometer.